A multiple-input multiple-output (MIMO) technology has been widely used in a wireless communications system to improve a system capacity and ensure cell coverage. For example, multi-antenna-based transmit diversity, open-loop/closed-loop spatial multiplexing, and demodulation reference signal (DMRS)-based multi-stream transmission are used in a downlink transmission of an long term evolution (LTE) system. DMRS-based multi-stream transmission is a main transmission mode in an LTE-advanced (LTE-A) system and a subsequent system. A procedure of the DMRS-based multi-stream transmission is as follows: user equipment (UE) first performs channel measurement according to a channel state information reference signal (CSI-RS) configured by an evolved NodeB (eNB). A measurement result includes a transmission rank, a precoding matrix corresponding to the transmission rank, and a channel quality indicator (CQI) corresponding to the transmission rank and the precoding matrix. Then, the UE feeds back the measurement result to the eNB. The eNB further performs downlink scheduling according to the measurement result fed back by the UE, and sends, according to a scheduling result, a physical downlink shared channel (PDSCH) to the UE by using a DMRS.
In a communications standard such as a current LTE-A Rel-13 standard, mechanisms for CSI reporting and feedback in three-dimensional MIMO may be classified into a mechanism 1 in which a CSI-RS resource is not precoded and a mechanism 2 in which a CSI-RS resource is precoded. In the mechanism 2, to eliminate impact caused by a movement of the UE and improve CSI feedback accuracy, a base station usually sends multiple sets of precoded candidate reference signal resources to the UE for selection and reporting. Different from the mechanism 1 in which only CSI reporting is performed, in the mechanism 2, extra selection needs to be further performed on multiple sets of precoded resources.
In the mechanism 1, beam selection is usually directly bound to a rank. For example, when the rank is 3 or 4, a precoding matrix and a phase rotation part that are corresponding to orthogonal beams need to be selected. However, in the mechanism 2, a precoding matrix for the multiple sets of precoded candidate reference signal resources is unknown to the UE, and in the mechanism 2, the UE may need to report multiple sets of candidate reference signal resources to the base station in one rank. In the mechanism 2, beam or resource selection usually has no direct association with a rank. Therefore, a manner in the mechanism 1 for selecting a candidate reference signal resource cannot be used in the mechanism 2, and there is an urgent need for an effective solution that can be used in the mechanism 2 to select a candidate reference signal resource.